During the formation of a semiconductor device many features such as word lines, digit lines, contacts, and other features are commonly formed over a semiconductor wafer. A goal of semiconductor device engineers is to form as many of these features in a given area as possible to increase yield percentages and to decrease manufacturing costs.
All heterogeneous structure on a semiconductor wafer requires lithography. Optical lithography, the lithographic method most used in leading-edge wafer processing, comprises projecting coherent light of a given wavelength from an illumination source (illuminator) through a quartz photomask or reticle having a chrome pattern thereon, and imaging that pattern onto a resist-coated wafer. The light chemically alters the photoresist and allows the exposed photoresist (if positive resist is used) or the unexposed photoresist (if negative resist is used) to be rinsed away using a developer.
With decreasing feature sizes, the limits of optical lithography are continually being tested and lithographic methods and materials are continually being improved through various developments, generally referred to as resolution enhancement techniques (RET's). RET's alter various aspects of the lithographic process to optimize the size and shape of a desired feature. For example, the wavelength of light used to expose the photoresist may be decreased, as longer wavelengths cannot resolve the decreasing feature sizes. The wavelength used with lithographic equipment has decreased from 365 nanometers (nm) in the mid-1980's to the current standard of 193 nm. Another RET includes optical proximity correction, which uses subresolution changes in the chrome pattern on the photomask or reticle to optimize the shape of the light focused on the photoresist. Without optical proximity correction, the chrome pattern is a scaled shape of the pattern which is to be produced. With very small features a scaled shape does not produce the desired pattern due to diffraction effects. However, the chrome photomask features can be modified in a manner that attempts to account for these diffraction effects. U.S. Pat. No. 6,245,468 by Futrell et al., assigned to Micron Technology, Inc. and incorporated herein by reference as if set forth in its entirety, describes an optical proximity correction apparatus and method. A third RET uses unequal photomask thickness of the quartz on which the chrome is formed at selected locations between the chrome to provide a phase-shift photomask. Phase shifting sets up destructive interference between adjacent light waves to enhance the pattern formed on the photoresist.
Another resolution enhancement technique is off-axis illumination, which improves the resolution of repeating patterns found in semiconductor device manufacture. FIG. 1 depicts an apparatus comprising off-axis illumination, and depicts an illuminator 10 comprising a laser which provides a coherent light source 12, an optical element 14, a zoom axicon 16, a first reflector 18, an optical homogenizer 20, a blade 22, a second reflector 24, a vertical photomask 26, a lens 28, and a wafer 30 comprising a layer of photoresist (not individually depicted).
In use, the coherent light 12 is output by the illuminator 10, which travels through the optical element 14. The optical element, which in FIG. 1 is depicted as a dipole element, directs the light in particular patterns of angles and improves the light pattern focused on the photoresist. In use, the dipole optical element 14 is used in FIG. 1 to expose a vertical photomask. It is then rotated 90° to expose the horizontal photomask 32 as depicted in FIG. 2. After exiting the optical element 14, the light reaches the zoom axicon which allows some control over the size and position of the light source 12. Depending on the equipment used, the direction of light output from the zoom axicon may be changed 90° by the first reflector 18. The optical homogenizer 20 normalizes the intensity of light across the coherent beam such that any granularity is removed. In some instances a blade 22 is used instead of the optical element 14, for example during testing of a particular pattern. A second reflector 24 may change the direction of the source 12 depending on the equipment used. The source 12 then reaches the chromed reticle, depicted as a vertical reticle in FIG. 1 and a horizontal reticle in FIG. 2, which determines the pattern which is focused through the lens 28 and onto the photoresist which covers the semiconductor wafer 30.
A structure similar to this, as well as the other RET's previous listed, are described and illustrated in A Little Light Magic, IEEE Spectrum, September 2003, pp. 34–39.
While a dipole element is depicted in FIGS. 1 and 2, other optical elements are used for various patterns in addition to the dipole element 14 depicted in FIG. 1. FIG. 3 depicts a quadrupole element 36 and an annular element 38.
Melioration to resolution enhancement techniques which would to improve the pattern produced on a photoresist layer during the formation of a semiconductor device would be desirable.